Process for recovering electrolytically produced alkali metal chlorates

ABSTRACT

Process for recovering alkali metal chlorates by electrolysis of alkali metal chloride solutions relies on high chlorate-tochloride ratios during electrolytic cell operation. Alkali metal chlorate is crystallized directly from the cell effluent using vacuum evaporative techniques.

United States Patent 1 OBrien et al.

[73] Assignee: Pennwalt Corporation, Philadelphia,

[22] Filed: July 6, 1973 [21] Appl. No.: 376,978

[52] U.S. Cl 204/95; 23/302 [51] Int. Cl C0lb 11/26 [58] Field of Search t. 204/95; 23/302, 300

[56] References Cited UNITED STATES PATENTS 2,5l l.5l6 6/1950 Schumacher 204/95 [451 May 13, 1975 3,043.75? 7/l962 Holmes 204/95 3,5l l,6l9 5/1970 Fuller et all 204/95 3,690.845 9/1972 Grotheer 23/302 FOREIGN PATENTS OR APPLICATIONS 754,132 3/l967 Canada 204/95 Primary Examiner-Oscar R. Vertiz Assistant E.raminerwayne A. Langel {57] ABSTRACT Process for recovering alkali metal chlorates by electrolysis of alkali metal chloride solutions relies on high chlorate-to-chloride ratios during electrolytic cell operation. Alkali metal chlorate is crystallized directly from the cell effluent using vacuum evaporative techniques.

5 Claims, 2 Drawing Figures STEAM 28 "Ha Vuc.

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PROCESS FOR RECOVERING ELECTROLYTICALLY PRODUCED ALKALI METAL CHLORATES This invention relates to a method for producing alkali metal chlorates, and more particularly relates to a process for direct crystallization of alkali metal chlorates from an aqueous solution containing alkali metal chlorates and chlorides.

Alkali metal chlorates are commonly produced by electrolysis of an aqueous solution of alkali metal chloride under conditions which produce a cell effluent containing both the chlorate and the chloride. The electrolysis is normally conducted commercially in diaphragmless electrolytic cells in which chlorine is produced at the anode while alkali metal hydroxide is formed at the cathode. The chlorine and hydroxyl ions are thus free to react chemically to form alkali metal hypochlorite as per the following equation.

2NaOH Cl NaCl NaOCl H O The hypochlorite converts rapidly to form chlorate in accordance with the chemical reaction:

3 NaOCl' 2 NaCl NaClO The methods previously employed for separating sodium chlorate from residual amounts of sodium chloride have generally been multi-step evaporating and concentratig techniques followed by a cooling to the temperature required for crystallization of the sodium chlorate. During the evaporation and concentration steps of the prior processes, the solubility of the sodium chloride is exceeded first and the solid crystals of the sodium chloride are removed by filtration or centrifugation. This salt is then redissolved and returned to the electrolytic process.

Inherent limitations on the solubility of sodium chloride, and the amount of water which must be fed into a cell along with the salt, do not permit the conventional graphite-anode cell to be operated at such concentrations whereby sodium chlorate can be recovered directly through cooling the effluent solution to a reasonable temperature. For example, a typical cell liquid concentration might be 400 to 450 grams sodium chlo rate per liter and I to 120 grams sodium chloride per liter. When graphite anodes are used the salt concentration must be kept at this high level in order to maintain current efficiency and prevent rapid wear of the anodes. The temperature required to produce solid chlorate from such solutions can be 0 C or lower.

Accordingly, prior commerical practice using graphite anodes first evaporated the water until the liquor became supersaturated with respect to sodium chloride whereupon the latter was crystallized out to produce solid salt. That is, while it is possible for either of the two major components, sodium chloride or sodium chlorate, of cell liquor to drop out of solution first, the usual combination of cell liquor concentrations and evaporation temperature and pressure has been such that salt is the first solid produced. This salt has then had to be redissolved for recycle back to the electrolytic process. The concentrated liquor, after salt removal, could thereafter be directed through asecond crystallizer operating at a lower temperature for recovcry of chlorate. As is apparent, the foregoing proccess required exacting control of conditions in both steps in order to achieve a successful result in the latter step.

With the advent of dimensionally stable anodes, i.e., metal electrodes of a valve metal, such as titanium, coated with a noble metal and/or oxide thereof, it has become possible to operate the cells over a wide range oftemperatures up to about the boiling point of the cell liquor and at chlorate concentrations far exceeding those of the conventional cell. Thus, it is now feasible to operate a cell sucessfully with good current efficiency at chlorate concentrations in excess of 700 grams of sodium chlorate per liter and with chloride concentrations as low as 40 grams sodium chloride per liter. Cell operations have been determined not to suffer greatly until the sodium chloride concentration is reduced to about 30 grams per liter. At the high chlorate-to-chloride concentrations thereby obtainable supersaturation by evaporation produces solid chlorate first if sufficient vacuum is applied.

Therefore, in accordance with the present invention, we provide a process for selectively crystallizing sodium chlorate first directly from an aqueous solution containing sodium chlorate and sodium chloride. The instant process consists essentially of (l) evaporation of water from the cell liquor, preferably under relatively high vacuum, until substantial amounts of solid alkali metal chlorate but no or little solid alkali metal chloride is formed, (2) mechanically separating the solids from the liquid such as by filtration or centrifugation, (3) washing of the solid chlorate crystals with cold water to remove occluded liquor and some solid impurities, and (4) recycling the liquid from the separation step back to the electrolytic cells along with fresh feed. The washings from step (3) may also be sent back to the operating cells.

The advantages attendant to crystallization of the alkali metal chlorate directly from the cell liquor include l) its great simplicity, (2) the ability to utilize conventional materials of construction, such as steel, in contradistinction to exotic materials, such as titanium, and especially (3) the greatly reduced evaporative load. With respect to the last mentioned advantage, it must be realized that except for the constant amount of water consumed in the cell and the amount carried out of the cell by the co-product hydrogen, any water brought into the process must be evaporated off. In the present invention, the only water which is added is that which is necessary to dissolve fresh salt for feed as compared to the conventional process in which water must be added to redissolve salt dropping out during the first crystallization step in addition to the fresh salt feed. As a result, the instant process is conducive to considerable steam economy, a significantly large cost factor in a chlorate process.

While prior systems have utilized an approach in which the alkali metal chlorate is crystallized out of the solution first, each of the earlier procedures suffers distinct drawbacks.

In U.S. Pat. No. 3,043,757, there is shown and described a process for electrolytic production of sodium chlorate in a boiling cell using very high dissolved solid concentrations with direct chlorate recovery in a cooling crystallizer. However, not only would a highconcentration boiling" cell be extremely troublesome in the event ofa power failure, in which case the solids would drop out as the cell cooled, but also the high temperature of operation requires more exotic materials of construction, the cell operation itself is troublesome, and it requires controls.

In US. Pat. No. 3,511,619, crystallization of alkali metal chlorate is effected from an alkali metal chloratechloride solution by adding sodium chloride and cooling the solution. However, this system has been found to produce an impure chlorate crystal because of contamination with the chloride.

In U.S. Pat. No. 3,690,845, a process is described in which alkali metal chlorate is crystallized out first from an electrolytically produced chlorate-chloride solution by introducing alkali metal hydroxide into the solution in an amount sufficient to depress the solubility of the alkali metal chlorate. Thereafter the solution is cooled from a temperature of about 80l()() C to about 25 to 40 C. However, this process requires the addition of caustic thereby augmenting the cost of operation together with the need to maintain a greater degree of care in pH and material balance controls.

It is therefore an object of this invention to provide a method for producing alkali metal chlorates from electrolysis of brine in which minimal evaporative loads are required, wherein the temperatures of operation are practical, and allowing for less imposing materials of construction with a greater degree of operating flexibility.

Other objects of this invention are to provide an improved process of the character described which is easily and economically accomplished, and highly efficient and effective in operation.

With the above and related objects in view, this in vention consists of the details of construction and combination of parts as will be more fully understood from the following detailed description when read in conjunction with the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating the process of the present invention.

FIG. 2 is an equilibrium phase diagram showing graphically the parameters of the present invention.

In the present invention, sodium chlorate may be produced by electrolysis of sodium chloride in electrolytic cells utilizing coated titanium anodes for example. The cells may be operated individually or in groups employing either series or parallel flow of the liquor therethrough so that the final cell liquor product contains at least 30 grams sodium chloride per liter of solution, and preferably at least 40 grams NaCl per liter. The weight ratio of sodium chlorate to sodium chloride should be at least 5 to l and preferably at least 7 to l. Concentrations of other materials such as dichromate, sodium hypochlorite, chlorine, etc., and cell operating conditions need only be consistent with sound practices.

The effluent from the cells is then subjected to vacuum evaporation in a crystallizer operating at a vacuum of at least 26 inches of mercury. The degree of evaporation is such that substantial amounts of sodium chlo rate are removed from the solution but little or none of the chloride is crystallized out. That is, the evaporation is performed below the solid phase NaClO -NaCl equilibrium curve of the phase diagram. The evaporation can be performed in a single vessel, or in a multiple effect evaporative system. However, it is preferable for reasons of steam economy to operate in a two-step evaporation system with vapor produced in the first vessel used as the source of heat in the second. In the latter instance, the initial step of evaporation is run at any convenient pressure level and in such a way that no solids would be produced.

The slurry produced in the crystallizer is then separated into solid and liquid fractions by mechanical means and the liquid fraction is returned to the cells for further electrolysis. The solids are washed with cold 5 water to provide clean, wet crystals of sodium chlorate which are then dried to yield the final product.

Referring now in greater detail to the drawings in which similar reference characters refer to similar parts, there is shown in FIG. 1 a process in which sodium chlorate may be recovered directly from an electrolytic solution of sodium chlorate and chloride without first crystallizing out solid salt.

In FIG. 2, graph A represents a typical course pursued by a process of prior commercial practice using graphite anodes. A cell liquor having a sodium chloride concentration of about 125 grams per liter (16 grams NaCl per 100 grams H and about 420 grams per liter of sodium chlorate (54 grams NaClO per 100 grams H O) would be subjected to evaporation (along line A1) until point Z (92 C) was reached, where saturation of sodium chloride would occur. Salt would then drop out of solution with further removal of water. Next the process would follow line A2 to point Y which represents the liquor concentration within the evaporator. A cooling crystallizer operating at about 45 C would cause the process to follow horizontal line A3 to point X and chlorate crystallizing from solution would be removed.

In the event one wished to produce chlorate directly from the cell liquor of the foregoing example without the crystallization of chloride also, the evaporative process along line Al would have to stop short of point W (equilibrium curve). It would be necessary to carry out the crystallization at 25 C to produce even a minute amount of chlorate. Achievement of the low temperature necessary for good recovery would be excessively expensive.

A representative process illustrative of the instant invention is demonstrated by graph B. Starting with a cell liquor concentration of about 75 grams of NaClO; per 100 grams H 0 and 8.2 grams NaCl per 100 grams H O, the solution would be subjected to vacuum evaporation of about 42 C and under a vacuum of about 28 inches of mercury. Evaporation would proceed along line B] until saturation of the sodium chlorate occurred at point V whereupon sodium chlorate would drop out. Thereafter, the curve would follow along line B2 until point T is reached, i.e., below the solid phase Na- ClO NaCl equilibrium curve. Point T represents a practical limitation of a vacuum evaporative crystallizer. That is, moving the curve VT to the right would not gain since a lesser amount of chlorate could be crystallized out before the line B2 ran into the solid phase NaClO NaCl equilibrium curve. While curve VT could be moved to the left by running under higher vacuum, this would involve unconventional equipment or exceedingly high operational expense.

By inspection of the coordinates of point T it can be determined that the ratio of the sodium chlorate to sodium chloride in the cell liquor must be at least :1 in order to produce solid chlorate under the chosen conditions. Therefore, the cell must operate to the right of RT. For good efficiency and voltage characteristic in addition to avoiding formation of undesirable perchlorate, the cell should operate with at least 30 gpl. sodium chloride. The dashed line RS at the lower portion of FIG. 2 is the straight line approximation for this minimum 30 gpl. NaCl limitation, the line RS being slightly inclined to designate the corresponding NaCl and Na- ClO concentrations in grams per 100 grams of water. i.e., corrected for density.

Point S itself is determined by the anticipated temperature of the cell room in the event of a power failure, for example. That is, if the cells were operating at a cell liquor concentration of 4.5 grams NaCl per l grams H 0 and l23 grams NaClO per I00 grams H O, chlorate would not drop out in the cells themselves unless the temperature were to fall below 50 C (122 F). Thus, the line TS represents outer limits of concentrations which would avoid crystallization of chlorate within the cells themselves should a shutdown occur. Therefore, the maximum chlorate-towhloride ratio would be represented by the slope of the line OS which passes through the origin, i.e., l23/4.5 or about 27:1.

While operation of the cells within the dashed triangle RST represents the outside limits of the present invention, the smaller dashed triangle QNP designates a set of conditions which are more restrictive but preferred. The line QN designates a minimum sodium chloride concentration of 40 grams per liter which would offer slightly improved voltage and current efficiency characteristics while providing some margin of safety for fluctuations and process shutdowns. Thus, point N, 98 grams NaClO per I00 grams H 0 and 5.8 grams NaCl per 100 grams H O, represents a temperature at 30 C (86 F) which would allow the cell room to drop to such a temperature without crystallizing out the chlorate of the designated concentration. The chlo rate concentration at point N is approximately 700 grams per liter and the slope of the line ON is 98/58 or a chlorate to chloride ratio of l6: 1. The slope of the line OP is approximately 7:1 which permits a more reasonable amount (e.g., over 25%) of chlorate to be recovered in each pass through the crystallizer.

The following example is illustrative of the mode of operation of the instant invention falling within the triangle N PQ.

EXAMPLE A cell was operated at about 600 gpl. NaClO and 50 gpl NaCl. The cell liquor contained gpl NaOCl and 5 gpl Na Cr O Anodic current density was 306 amps/ft. and the temperature of the cell was 90 F. The anode was titanium coated with platinum. Current eff"- ciency was 92%. Chlorine was added to the recirculating liquor periodically to maintain pH below 7.0.

A sample of liquor was fed into a rotary vacuum evaporator operating at 28 inches of mercury vacuum. The flask of the evaporator was heated with warm water at 105 F. Condensate was collected in a graduate, and evaporation discontinued when the proper amount has been collected. The mixture was then cooled to about 60 F, settled and decanted. The liquor analyses were as follows:

The chlorate crystals was washed and dried giving a recovery of more than 60 percent of the chlorate in the sample with quality equivalent to normal commercial product.

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting, since the invention may be variously embodied without departing from the spirit thereof, and the scope of the invention is to be determined as claimed.

What is claimed is:

l. A method for electrolytically producing alkali metal chlorate from an alkali metal chloride comprising the steps of:

a. electrolyzing an aqueous sodium chloride solution at a temperature above about 50 C until an unsat urated cell liquor is produced having a sodium chlorate to sodium chloride ratio of between 5:] and 27:1, the concentration of the sodium chloride component being maintained in the range between a lower level above about 30 grams per liter and an upper level below which sodium chloride cannot crystallize,

b. withdrawing the electrolyzed liquor from the cell as effluent and while above 50 C subjecting the effluent to evaporative concentration under reduced pressure and at constant temperature to effect crystallization of pure sodium chlorate crystals from the liquor, and

c. separating the pure sodium chlorate crystals from the liquor.

2. The method of claim I wherein said chlorate crystallization during evaporative concentration is performed under vacuum.

3. The method of claim 2 wherein the evaporative concentration is performed at a vacuum level in excess of 26 inches of mercury.

4. The method of claim 2 wherein the chloride concentration is in excess of 40 grams per liter and the chlorate-to-chloride ratio is between 7:1 and 16:1.

5. The invention of claim 2 wherein the evaporative concentration is multiple effect. 

1. A METHOD FOR ELECTROLYTICALLY PRODUCING ALKALI METAL CHLORATE FROM AN ALKALI METAL CHLORIDE COMPRISING THE STEPS OF: A. ELECTROLYZING AN AQUEOUS SODIUM CHLORIDE SOLUTION AT A TEMPERATURE ABOVE ABOUT 50*C UNTIL AN UNSATURATED CELL LIQUOR IS PRODUCED HAVING A SODIUM CHLORATE TO SODIUM CHLORIDE RATIO OF BETWEEN 5:1 AND 27:1, THE CONCENTRATION OF THE SODIUM CHLORIDE COMPONENT BEING MAINTAINED IN THE RANGE BETWEEN A LOWER LEVEL AVOVE ABOUT 30 GRAMS PER LITER AND AN UPPER LEVEL BELOW WHICH SODIUM CHLIRIDE CANNOT CYRSTALLIZE, B. WITHDRAWING THE ELECTROLYZED LIQUOR FROM THE CELL AS EFFLUENT AND WHILE ABOVE 5O*C SUBJECTING THE EFFULENT TO EVAPORATIVE CONCENTRATION UNDER REDUCED PRESSURE AND AT CONSTANT TEMPERATURE TO EFFECT CRYSTALLIZATION OF PURE SODIUM CHLORATE CRYSTALS FORM THE LIQUOR, AND C. SEPARATING THE PURE SODIUM CHLORATE CRYSTALS FORM THE LIQUOR.
 2. The method of claim 1 wherein said chlorate cryStallization during evaporative concentration is performed under vacuum.
 3. The method of claim 2 wherein the evaporative concentration is performed at a vacuum level in excess of 26 inches of mercury.
 4. The method of claim 2 wherein the chloride concentration is in excess of 40 grams per liter and the chlorate-to-chloride ratio is between 7:1 and 16:1.
 5. The invention of claim 2 wherein the evaporative concentration is multiple effect. 